(1) Field of the Invention
This invention relates to a heat-sensitive recording medium, and more specifically to a heat-sensitive recording medium useful in the practice of the thermal ink-transfer recording or sublimation ink-transfer recording method.
(2) Description of the Prior Art
It has heretofore been known to have a dye or pigment carried along with a binder resin on one side of a sheet-like base material such as polyester film to form a heat-sensitive recording layer and to heat the heat-sensitive recording layer in a desired pattern by way of the back side of the base material to transfer the ink onto a material to be printed. It has also been known to use a thermally-sublimable dye as the above dye and to transfer the dye alone in a similar manner onto a material to be printed.
Since thermal energy is applied through the back side of a sheet-like base material in such conventional methods, the back side of the sheet-like base material is required to have sufficient heat resistance so that a thermal head does not stick on the back side.
It has hence been practised in the prior art to form a layer of a resin having relatively good heat resistance, for example, a polyurethane resin, acrylic resin, modified cellulose resin or a mixture thereof on the back side of a sheet-like base material of a heat-sensitive recording medium.
Although such conventional heat-sensitive recording media are provided on the back sides thereof with a heat-resistant layer of such a resin as mentioned above, these resins are thermoplastic and do not have sufficient resistance to heat. They are thus accompanied by a drawback that they tend to stick a thermal head to render insufficient the release of the thermal head from the heat-sensitive recording medium.
It has been attempted to incorporate an inorganic filler such as talc or fluoroplastic powder in such a heat-resistant layer with a view toward providing a solution to these drawbacks. Heat-resistant layers containing such an additive are however accompanied by drawbacks that due to the existence of such powder on their surfaces too, thermal heads are subjected to considerable smearing and wearing and their service life is shortened substantially in spite of their high price.
These various drawbacks can be solved by using a resin whose softening point is very high. There is however no suitable solvent for so-called heat-resistant resins known to date, so that difficulties still remain regarding their application on sheet-like base materials. Even if they can be applied, layers formed of these conventional heat-resistant resins have insufficient adhesion to sheet-like base materials and moreover are hard and brittle. It has hence been unable to form heat-resistant layers equipped with sufficient flexibility.
It has hence been desired to develop a resin having not only excellent flexibility but also superb heat resistance so that the above-mentioned problems can be solved.